This invention relates to an optical plastics material which is a blend of two kinds of polymers different in refractive index. One of the polymers is a methacrylic resin and the other a copolymer of fluoro-compounds.
Some kinds of plastics have long been used as optical materials for lenses, prisms. etc. Also in recently developing optical fibers for light communications and for displaying and advertising purposes optical plastics are attracting great interest. Particularly for transmission of light over relatively short distances the role of plastics optical fibers is important since optical fibers of this category are superior in flexibility and workability to the popular quartz optical fibers though inferior in respect of transmission loss.
Plastics optical fibers currently produced on the commercial basis are limited to the step-index type fibers, which consist of a plastics core that exhibits a relatively high index of refraction and a plastics cladding that exhibits a lower index of refraction. As the core material acrylic resins represented by poly(methyl methacrylate) is prevailing. The cladding material is usually selected from fluoropolymers as will be understood from the degree of atomic refraction. Examples are polymers of fluorine-substituted acrylates or methacrylates and vinylidene fluoride base copolymers.
Researches have also been conducted on optical plastics materials of the graded-index type in which the refractive index decreases gradually from the central axis radially toward the periphery. Such materials are essential for the development of the graded-index plastics fibers for transmission of light and, besides, will be valuable for special lenses.
As to means of producing plastics optical materials of the graded-index type or desirably controlling the refractive indices of plastics materials, trials have been given to graft copolymerization, photocopolymerization, modification of conventional polymers by chemical reaction and blending of polymers different in refractive index. Among these methods, blending of a polymer relatively high in refractive index with another polymer lower in refractive index is deemed simple, easy and widely applicable. However, this method has rarely provided practicable optical plastics. The main reason for the unsuccess is that two kinds of polymers greatly different in refractive index are in most cases poor in compatibility or mutual solubility, so that a blend of such polymers becomes low in mechanical strength and also in transparency when moulded or extruded into a desired shape.